It is necessary at present to combine closure devices for vehicle tank filler necks with additional, separate fluid switching elements in order to cover all functions and operational states of a vehicle tank. For example, an underpressure or overpressure valve is required in order to ventilate the vehicle tank for pressure equalization. Responsible treatment of the environment and recent requirements of the law have furthermore accelerated the development of so-called ORVR systems (On-board Refueling Vapor Recovery System), by means of which the fuel vapors created during filling--independently of a miscellaneous removal via the gas displacement line of the gas pump--as well as the vapors escaping during vehicle operation via the overpressure valve are caught in the vehicle ("on-board") in a recovery container provided with an activated charcoal filter. During vehicle operation the recovery container is washed by the combustion feed gases, which remove the fuel residue from the activated charcoal filter and furnish it to the combustion chamber. Therefore further valves are required, which during fill-up release appropriate aspirating and recovery openings and during vehicle operation release appropriate pressure equalization openings.
The latest regulations furthermore require a completely pressure-free operation of the vehicle tank in order to further reduce the emission of fuel vapors. The overpressure and underpressure valves used up to now for the pressure-equalized connection to the recovery container can therefore no longer be used and must be replaced by separate switching valves, which release the pressure equalization connection during vehicle operation.
The already widely used employment of a so-called overfill valve, which prevents the overfilling of the tank as far as into the filler neck, further increases the number of fluid switching elements.